1. Field of the Invention
The present invention relates to wireless data telemetry system utilizing spread spectrum frequency hopping transceivers, and, more particularly, to circuits and methods for selectively controlling modulation depth when transmitting data.
2. Discussion of the Prior Art
Wireless data telemetry methods have been well documented; infrared and radio frequency (RF) are the principal wireless communication technologies described in the prior art. Infrared beam communications systems cannot operate over distances of more than a few feet and so are limited to applications such as bar code scanning, local computing devices, and television (or other home appliance) remote control.
Accordingly, most wireless data transmission products in the prior art utilize standard RF technology, i.e., radios, the same technology used in vehicle dispatch and police communication systems. Standard RF products are relatively simple and inexpensive to build but require FCC licenses to operate. RF transmissions are susceptible to interference from a growing number of sources and to interception by readily available eavesdropping equipment. The unreliable quality of standard RF transmissions makes the technology unsuitable for applications where all of the information transmitted must be accurate, complete, and secure.
In order to overcome the shortcomings of standard RF transmission, direct sequence spread spectrum (DSSS) was developed. DSSS radios divide or slice transmissions into small bits thereby spreading energy from the bits simultaneously across a wide range or spectrum of radio frequencies. DSSS is a relatively unreliable transmission medium, however, because spreading the message across a wide spectrum greatly reduces the strength of the radio signal carrying the message weakening the signal. A DSSS receiver must simultaneously monitor the entire allotted spectrum, thereby risking severe interference from any high energy RF source appearing within the monitored spectrum. DSSS performance also degrades quickly in shared-service environments having multiple radio systems operating simultaneously.
Frequency hopping spread spectrum (FHSS) technology was developed by the U.S. military to prevent interference with or interception of radio transmissions on the battle field and is employed by the military in situations where reliability and the coexistence of multiple radio systems are critical. Standard RF and DSSS cannot match the reliability and security provided by frequency hopping. Instead of spreading (and therefore diluting) the signal carrying each bit across an allotted spectrum, as in DSSS, frequency hopping radios concentrate full power into a very narrow spectral width and randomly hop from one frequency to another in a sequence within a defined band, up to several thousand times per second. Each FHSS transmitter and receiver coordinate their hopping sequence to ensure a reliable exchange of data. Upon encountering interference on a particular frequency, the transmitter and receiver retain the affected data, hop to another point in the spectrum and then continue the transmission. Statistically, there are always spaces without interference somewhere in the spectrum, since benign producers of interference or a hostile jammer cannot easily jam all possible frequencies simultaneously and at high power radiation levels.
The frequency hopping transmitter and receiver find frequencies with no interference and complete the transmission. This ability to avoid interference enables FHSS radios to perform more reliably over longer ranges than standard fixed channel RF or DSSS radios. In the past, frequency hopping communication systems have been used almost exclusively in extremely expensive robust military or government communication systems.
Generally speaking, data telemetry is the transmission of short packets of information from equipment or sensors to a recorder or central control unit. The data packets are transferred as electric signals via wire, infrared or RF technologies and data is received at a central control unit such as a computer with software for automatically polling and controlling the remote devices. The control unit analyzes, aggregates, archives and distributes the collected data packets to other locations, as desired, via a local area network (LAN) and/or a wide area network (WAN). Wireless data telemetry provides several advantages over data telemetry on wired networks. First, wireless systems are easier to install; second, installation and maintenance costs can be much lower, and third, operations can be reconfigured or relocated very quickly without consideration for rerunning wires. Fourth, wireless telemetry offers improved mobility during use.
The Federal Communications Commission (FCC) has designated three license-free bandwidth segments of the radio frequency spectrum and made them available for industrial, scientific and medical (ISM) use in the United States. These three segments are 900 MHZ, 2.4 GHz and 5.8 GHz. Anyone may operate a wireless network in a license-free band without site licenses or carrier fees, and is subject only to a radiated power restriction (i.e., a maximum of one watt radiated power). The radio signals transmitted must be spread spectrum. Foreign national spectrum regulation organizations and international telecommunications bodies have also agreed to recognize a common license-free ISM frequency at 2.4 GHz, and so a defacto international standard for license-free ISM communications has emerged. The ISM band at 2.4 GHz provides more than twice the bandwidth capacity (and is subject to far less congestion and interference) as compared to the ISM band at 900 MHZ. Several industrial nations do not permit a license-free ISM band at 900 MHZ and relatively few nations have a license-free ISM band of 5.8 GHz. However, the United States, Europe, Latin America and many Asian countries have adopted an ISM band at 2.4 GHz, the only band which so many nations offer for license-free operations.
What is needed, then, is an inexpensive, easy to use and robust data telemetry and communication system which will dynamically establish and maintain communication links, preferably operating in the common license-free ISM frequency band and providing reliable communications for a variety of users in commercial and industrial environments.
The applicant has recognized that spread spectrum transceivers pose a number of challenges not addressed by vendors of off-the-shelf RF components. For example, when switching from receive mode to transmit mode, an appropriate Voltage Controlled Oscillator must be activated and input to a modulator for transmitting data over a wide range of frequencies. Often, the modulator input sensitivity varies with transmit frequency, thus leading to a variation in modulation index for the transmitted data, with some frequencies having less than optimal modulation depth. Sub-optimally modulated data can diminish overall system performance by severely limiting range. The receiver of poorly modulated data has less signal to noise ratio margin for those frequencies subject to sub-optimally modulated data.
In accordance with the present invention, a frequency hopping spread spectrum communication system is adapted to dynamically establish and maintain communication links. The system is economically implemented at 2.4 GHz, provides an optimum balance between data rate and range and provides 9.6 kilobits per second (9.6 Kbps) data transmission over an outdoor line of sight range of approximately 35thousand meters. The communication system includes components ideally suited to specific wireless data telemetry applications. A transceiver is configured on two printed circuit cards and includes RF and computer control components in a compact package approximately the size of a deck of cards. Each transceiver includes a shielded RF board or module with a frequency hopping transmitter and receiver, an antenna, and a digital control board or module. An application interface is included to communicate with specific OEM products utilizing serial (transistor/transistor logic, TTL) or other standard interfaces. The transceiver transmits or receives on any of 550 independent, non-interfering frequencies and functions as a half duplex, bi-directional communication device. The transmit and receive functions are time interleaved in a non-overlapping fashion, consistent with the requirements of a frequency hopping radio. The transmit interval is restricted to less than 0.4 seconds on any particular frequency within a 30 second interval. In the course of a normal information exchange, a given transmission is generated on a frequency selected from a set of all available hop frequencies. The transmission is limited in duration to the availability of incoming data, and following the transmission, the radio switches to a receive mode and processes any incoming data. Once reception is complete, the transmit interval/receive interval cycle is restarted on a new frequency selected from the hop frequency set. Transmit receive cycling continues until all 75 unique frequencies in the set have been used, whereupon the frequency selection process reenters the top of the table and begins reusing the same 75frequencies.
Transmitted data is directly modulated onto a synthesized carrier by use of minimum shift keying (MSK) modulation. First and second local oscillators (LOs) are controlled in frequency by use of a single loop indirect frequency synthesis. Samples of both first and second voltage controlled oscillators (VCOs) are divided down using phase-locked loop integrated circuit elements, where each sample is compared to an onboard 8 MHZ crystal reference oscillator. During the transmit interval, a single transmitter VCO is controlled by the same device and in the same manner.
The basic transmitted signal is generated by a voltage-controlled-oscillator (VCO) that operate""s in the 2.4 to 2.4835 GHZ frequency band. The signal is then amplified by three stages of amplification. All three amplification stages and the VCO are switched ON for transmit and switched OFF for receive.
The frequency synthesizer generates the modulated transmit signal which is phase locked to the on-board 8 MHZ reference oscillator. The 8 MHZ reference is a crystal oscillator that is controlled by the off-board microprocessor. To enable a cost effective solution for the reference oscillator, an inexpensive crystal is utilized. Because a frequency tolerance of 3 parts per million (ppm) must be maintained for the transceiver to communicate, a frequency compensation routine is programmed in the microprocessor. The compensation deals with both the initial crystal manufacturing tolerance and maintaining tolerance over the specified xe2x88x9220 to 60 degrees Celsius temperature range.
The transmitted signal is generated by a VCO that is switched on during transmit and operates over a 350 MHZ tuning range roughly centered on 2.44 GHz. During operation, the VCO only tunes in the 2.4 to 2.4835 GHz band. Having a larger tuning range allows for manufacturing tolerances without the need to tune the oscillators for each manufactured board. During operation, the synthesizer chip is programmed to the required hop frequencies.
As noted above, it was recognized that when switching from receive mode to transmit mode using a classical phase-locked-loop (PLL) circuit, the modulator input sensitivity varied with transmit frequency, thus leading to a variation in modulation index for the transmitted data, with some frequencies having less than optimal modulation depth. In order to overcome this problem, a circuit was developed for use between the source of transmit data, the micro processing unit (MPU), and the transmit PLL summing junction. The circuit is called a programmable attenuator, which provides a selectively attenuated signal that is coupled into the PLL summing junction to generate a modulated data signal for transmission.
The programmable attenuation circuit dynamically adjusts the transmit data stream voltage level required to achieve a fixed level in frequency deviation based on the current operating frequency. Each VCO will provide a specific change in frequency for a given change in input control voltage signal; the change will be dependant, in part, on the present, pre-adjustment frequency. In accordance with the method of the present invention, the specific change in frequency for a given change in input control voltage signal is predicted after first having obtained a complete set of VCO characterization measurements. Those characterization measurements are stored in memory and the MPU is programmed to control the programmable attenuation circuit, providing the exact selectively attenuated data signal level required for the VCO input control signal at the required frequency.